JAVA Study Notes - summarized articles (a)

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JAVA Study Notes - summarized articles (a)

A, Java Overview

• JVM (Java Virtual Machine): Java Virtual Machine. Java programs run directly in the JVM, so to ensure the cross-platform features of Java. JVM Java programs and act as a bridge platform.
• JRE (Java Runtime Environment): Java runtime environment, including the JVM + Java core class libraries.
• JDK (Java Development Kit): Java development tools, including jre + development tools. Developers using the JDK to compile, debug, and execute Java programs.

Two, Java data types

java data types can be divided into two categories: basic data types and the reference data type.

Types of	Keyword	Occupation and byte-digit	Value range
Byte	byte	Occupies 1 byte (i.e. 8 bits)	-128 to 127 (i.e.: -2 . 7 ~ 2 . 7 - 1)
Short integer	short	2 bytes (i.e., 16)	-32 768 to 32 767 (i.e.: -2 15 to 2 15 - 1)
Integer	int	Occupies 4 bytes (32-bit)	~ -2147483648 2147483647 (i.e.: -2 31 is ~ 2 31 is - 1)
Long integer	long	It occupies 8 bytes (i.e., 64)	-9223372036854775808 ~ 9223372036854775807 (i.e.: -2 63 is ~ 2 63 is - 1)
Single-precision floating-point number	float	Occupies 4 bytes (32-bit)	Absolute value: 1.4e-45 ~ 3.4e + 38
Double-precision floating-point number	double	It occupies 8 bytes (i.e., 64)	Absolute value: 4.9e-324 ~ 1.8e + 308
Character	char	2 bytes (i.e., 16)	0 to 65,535 (i.e., 0 to 2 16 -1)
Boolean	boolean	Occupies 1 byte (i.e. 8 bits)	true and false

Third, the basic concept

• Keywords : also known as reserved words, for example: byte, char, break, for , this, super, final, class, extends , and so on.

• Identifier : Java package name, class name, an interface name, method name, object name, constant names, variable names, etc. are collectively referred to as an identifier.

• Operators :
  arithmetic operators: +, - , /,%
  increment decrement operators: +, - -
  relational operators:>,> =, <=, ==,! =
  Logical operators:! , &&, ||
  bitwise operators: ~, &, |, ^
  shift operator: <<, >>, >>>
  assignment operator: =, + =, - =, =, / =,% = | = ,! =, << =, >> =
  conditional operator: ?:, Ternary operator.
  (Ps: note operator precedence)

• Java program structure, there are three basic types : sequential structure, selection structure, cyclic structure.
  Order structure: that is, the program statements written order from top to bottom, executed one by one.
  Select structure: also known as branch structure, such as if statements, switch statements.
  Looping structures: For "statement is executed repeatedly" loop can be used to write. For example, for statement, while statement, do ... while statement.

• Array : is a reference type (i.e. type Object), an ordered set of data by a number composed of the same type, each of which is called a data element.
  Array initialization: at the same time declare the array, the array elements and then allocate space assigned. E.g:

int a[] = {1 , 2 , 3};//直接给每个元素（成员）赋值
int b[] = new int [3];//创建了长度为3的数组对象，每个元素取默认值，例如int类型为0
b[0] = 1;b[1] = 2;b[2] = 3;//给b数组的每个元素赋值

• An array of objects : an object is an array of array elements.
  E.g:

public class Teacher{
    public static void main(String[] args) {
        Student[] student = new Student[2];//实例化对象
        student[0] = new Student(1,"张三");//给数组元素赋值
        student[1] = new Student(2,"李四");
        //遍历数组访问元素
        for(int i = 0;i < student.length;i++){
            System.out.println(student[i].id+" "+student[i].name);
        }
    }
}
class Student{
    int id;
    String name;
    //有参构造方法
    public Student(int id,String name){
        this.id = id;
        this.name = name;
    }
}